This invention relates to magnetrons used for microwave application, especially to magnetrons having an anode-cathode structure in which the number of resonant cavities comprising the anode of the magnetron can be reduced and stable microwave output can be obtained.
Since magnetrons can generate microwave power efficiently, they are used widely for radar equipment, medical devices, cooking devices such as microwave ovens, and other microwave devices.
Such magnetrons comprise a body having resonant cavities and a microwave output unit, and a filter that suppresses microwaves leaking from a power supply portion that supplies power to the body.
The number of resonant cavities is usually twelve which are formed by twelve anode vanes. Recently, a ten-cavity resonator formed by ten anode vanes was proposed.
FIG. 12 shows a cross section of a conventional magnetron structure example. 1 is a filament, 2 is anode vanes, 3 is an anode cylinder, 4 and 4' are permanent magnets, 5 and 5' are pole pieces, 6 and 6' are yokes, 7 is an antenna lead, 8 is an antenna, 9 is an exhaust tube, 9a is a concave portion formed by sealing off the exhaust tube 9, 10 is an antenna cover, 11 is a cylindrical insulator, 12 is an exhaust tube support, 21 is an upper end shield, 21' is a lower end shield, 23 and 24 are filament leads, 25 is an input side ceramic body, 126 is a filament terminal, 27 is a spacer, 28 is a sleeve, 31 is a choke coil, 32 is a feed-through type condenser. 33 is a filter case, 34 is a cover, 35 and 35' are sealing parts, 41 is an upper sealing part, 42 is a lower sealing part, 43 is a metallic gasket, and 45 is a cooling fin.
In FIG. 12, plural anode vanes 2 are arranged radially around the cathode filament 1 to form resonant cavities. These anode vanes 2 are brazed to the anode cylinder 3 or press-formed integrally with the anode cylinder.
Above and under the anode cylinder 3 are arranged magnetic pole pieces 5 and 5', formed of high permeability material such as soft-iron, and cylindrical permanent magnets 4 and 4'. The magnetic fluxes generated from the permanent magnets 4 and 4' pass through the magnetic pole pieces 5 and 5' and into the interaction space formed between the cathode filament 1 and the anode vanes 2 to provide an axial DC magnetic field.
The yokes 6 and 6' form a magnetic circuit for the magnetic fluxes from the permanent magnets 4 and 4' with the magnetic pole pieces 5 and 5'.
The electrons emitted from the cathode filament 1 at a negative high potential make a circular motion under the electric field and the direct magnetic field and generate a microwave field in each anode vane 2.
FIG. 13 shows a top view of the conventional magnetron anode structure shown in FIG. 12. 2a and 2b are cutouts formed in anode vanes 2 and 2'. 61 is a first strap ring and 62 is a second strap ring. The same parts as those shown in FIG. 12 are given the same reference numerals in this figure.
In FIG. 13, the anode vanes are divided into two groups, which are arranged alternately as 2 and 2'. These anode vanes 2 and 2' are radially arranged from the inside wall toward the axis 0 of the anode cylinder 3.
The anode vanes 2 and 2' are connected to each other alternately through first and second annular strap rings of different diameters 61 and 62, respectively, soldered to vanes at the cutouts thereof 2a and 2b. These strap rings are also arranged at the bottom ends of the anode vanes in the same way.
FIGS. 14(a) and 14(b) illustrate the strap rings shown in FIG. 13. FIG. 14 (a) shows a perspective view of the first strap ring of a small diameter, and FIG. 14 (b) shows a perspective view of the second strap ring of a large diameter. As shown respectively in FIGS. 14 (a) and 14 (b), both the first and second strap rings 61 and 62 are annular, but rectangular in cross section.
FIG. 15 shows a cross section of an essential part of an anode vane to explain how the anode vanes are connected to the strap rings. The same parts as those shown in FIGS. 12 through 14 are given the same reference numerals in this figure.
The small diameter strap ring 61 is in contact with the cutout 2a of this anode vane 2, while the large diameter strap ring 62 is not in contact with it. One of the anode vanes shown in FIG. 13 is silver-soldered to an antenna lead 7 (FIG. 12) used to receive microwaves.
The microwave field generated in the resonant cavities formed by the anode vanes 2 and 2' is led by the antenna lead 7 to the antenna 8, then transmitted through the antenna cover 10 that protects the antenna 8. The antenna 8 is integrated with a choke 9 used to prevent unnecessary radiation.
In general, the cathode filament that emits electrons is made of tungsten that contains a very small quantity of thorium oxide (ThO.sub.2), in view of the electron emissivity, workability, etc.
The upper end shield 21 and the lower end shield 21' are supported by the cathode leads 23 and 24. In general, molybdenum (Mo) is adopted for these end shields and the filament leads, considering heat resistance and workability. The two filament leads 23 and 24 are supported by input side ceramic 25. The filament leads 23 and 24 are silver-soldered vacuum-tight to the input side ceramic 25 via the terminal plates 26.
When vibration, shock, or something similar is applied to the magnetron, the filament leads 23 and 24 vibrate. In addition, since the vibration mode differs between the filament leads 23 and 24, a mechanical stress is generated in the cathode filament 1. This may cause the cathode filament 1 to snap in some cases.
The spacer 27 is used to prevent this snapping of the filament 1. This spacer 27 is very effective to make both the filament leads 23 and 24 vibrate almost in the same mode when the filament leads vibrate. As a result, almost no stress is applied to the cathode filament. The sleeve 28 is used to fix the spacer 27 at the desired position.
FIG. 16 shows a cross section of a conventional magnetron structure to explain another embodiment. 26 is a terminal plate. The same parts as those shown in FIG. 12 are given the same reference numerals in this figure.
In FIG. 16, plural anode vanes 2 are brazed to the anode cylinder 3 around the helically coiled cathode filament 1, or plural anode vanes 2 are press-formed integrally with the anode cylinder 3.
Above and under the anode cylinder 3 are arranged the magnetic pole pieces 5 and 5', made of high permeability material such as soft iron, and annular permanent magnets 4 and 4'.
The magnetic fluxes generated from the permanent magnets 4 and 4' pass through the magnetic pole pieces 5 and 5' and into the interaction space formed between the cathode filament 1 and the anode vanes 2 and provide an axial direct magnetic field.
The yokes 6 and 6' constitute a magnetic circuit for the magnetic fluxes from the permanent magnets 4 and 4' with the magnetic pole pieces 5 and 5'. The electrons emitted from the cathode filament 1 at a negative high potential make a circular motion under the electric field and the direct magnetic field and generate a microwave field in each anode vane 2.
The generated microwave field reaches the antenna 8 through the antenna lead 7, then it is outputted to external devices via the antenna cover 10.
The cathode filament 1 is supported by the upper end shield 21, the lower end shield 22, and the filament leads 23 and 24. The filament leads 23 and 24 are connected to the leads 23' and 24' which are connected to the choke coil 31, via the terminal plates 26 that are silver-soldered to the top surface of the input ceramic body 25.
On the underside of the magnetron body are provided the filter case 33, that encases both the choke coil 31 and the feed-through type condenser 32, and the cover 34 for the filter case.
The choke coil 31 connected to the leads 23' and 24' forms an L-C filter together with the feed-through type condenser 32 to suppress the low frequency components coming out via the cathode leads. High frequency components are shielded by the filter case 33 and the cover 34 for the filter case.
The cooling fins 45 arranged at the outer periphery of the anode cylinder 3 radiate the heat generated in the magnetron operation. FIG. 17 shows a top view of the conventional magnetron anode structure shown in FIG. 16. The same parts as those shown in FIG. 16 are given the same reference numerals in this figure.
In FIG. 17, anode vanes are divided into two groups 2 and 2', which are arranged alternately. The anode vanes 2 and 2' are arranged radially from the inside wall toward the axis of the anode cylinder 3.
The anode vanes 2 and 2' are connected alternately by first and second annular strap rings 61 and 62 at the upper and lower ends of the vanes, that is, the antenna-lead-side end and the filament-lead-side end. The first and second strap rings 61 and 62 are different in diameter from each other.
FIGS. 18 (a) and 18 (b) illustrate the strap rings shown in FIG. 17. FIG. 18 (a) shows a top view and a cross section of the first strap ring 61 of a small diameter. FIG. 18 (b) shows those of the second strap ring 62 of a large diameter. Strap rings 61 and 62 are connected to the anode vanes at the projections 61a or 62a formed at their inner or outer periphery as depicted in FIGS. 18(a) and 18(b), respectively.
FIG. 19 shows the cross section of the anode cylinder taken along line XIX--XIX in FIG. 17. The anode vanes 2 and 2' are connected alternately to each other through the strap rings 61 and 62, and the strap rings 61' and 62' shown in FIGS. 18 (a) and 18 (b).
Other parts in the configurations are all the same as those shown in FIG. 12, so the explanation is omitted here.
This kind of magnetron is disclosed, for example, in Japanese Utility Model Publications No. 56504/1982 and No. 25656/1988.
The conventional magnetron for microwave ovens adopts the above basic structure. The conventional twelve-anode-vane magnetron has to employ a large-diameter and thick-wall anode cylinder, resulting in use of a large amount of copper. Therefore, it is desirable to reduce the number of anode vanes and the cylinder diameter to provide material saving.
Japanese Utility Model Publication No. 25656/1988 disclosed a compact-cylinder anode magnetron that uses ten anode vanes. This magnetron for use in microwave ovens is intended for reduction of the size and weight of the tube, while preventing the Q of the cavity resonator from degrading, improving the overall power efficiency, and reducing the line noise.
The compact magnetron adopts a ratio F/G of the outer cathode diameter F to the diameter G between the radially internal ends of the anode of vanes in the range of F/G=0.38 to 0.47, to obtain stable microwave power, the practical diameter G between the radially internal ends of the anode vanes being 8.09 to 10.0 mm, and the practical outer cathode diameter F being 3.62 to 4.02 mm, but these practical values of G and F do not permit sufficient material savings.
This magnetron satisfies efficiency, load stability, and other properties required for use in microwave ovens, except for reduction in the size of the magnetron tube. When the number of anode vanes is reduced from ten to eight to further reduce the size of the magnetron tube, however, it becomes difficult to obtain microwave power output of several hundreds of watts efficiently and stably.
The frequency range prescribed for magnetrons used for microwave ovens are 2400 to 2500 MHz. And, the basic oscillation frequency of the magnetron is a 2450 MHz band. The microwave power outputs of magnetrons is usually a few hundreds of watts to 1,000 watts and is used mainly for home microwave ovens. However, since the actual oscillation spectrum has a band-like distribution, the frequency spectrum of magnetrons has insufficient margin for allowable microwave leakage value for the range of the 2400 to 2500 MHz. The performance required for the eight-vane compact-anode magnetron is as shown below. In order to obtain this performance, the band width of the oscillating frequency spectrum should be narrowed and microwave leakage should be reduced.
(1) The magnetron oscillates at the basic frequency of 2450 MHz in the .pi. mode. When the magnetron stability is low, however, the oscillation may become unstable. In other words, it may oscillate at a frequency band beyond 2450 MHz band. Especially, in the case of home microwave ovens, the load is food, and the load impedance changes significantly according to the weight and shape of the food. Thus, a well-stabilized magnetron is needed.
(2) The efficiency of microwave ovens (microwave output/input) is 50 to 55%. The magnetron oscillation efficiency to Obtain this efficiency must be about 70%. Thus, an efficiency of about 70% is required even when the number of vanes is reduced.
(3) For the magnetron used for microwave ovens, foods vary from heavy load foods to light load foods when viewed from the magnetron. Especially, for light load foods, the microwave power absorption is less, so most of the microwave power returns to the magnetron. The microwave returned to the magnetron is consumed by anode vanes, which results in an increase in the temperature of those vanes. To avoid this, therefore, the increase in the temperature of vanes must be suppressed by improving the thermal allowance of the vanes.
(4) The higher the pulling figure of a magnetron is, the more easily the magnetron can be used. In other words, the higher the magnetron pulling figure is, the more easily the microwave output can be obtained for a wide range of load impedances. This makes it easier to design cooking chambers for microwave ovens, which is the load for the magnetron. The pulling figure of the magnetron indicates a degree of ease in supplying microwave power output to a load. The higher the value is, the larger the microwave power that is supplied to the load; therefore the larger the allowance for designing an impedance becomes for a microwave oven, for instance. However, in general, the higher this pulling figure becomes, the lower the operating stability of the magnetron (peak anode current for maintaining a 2450 MHz band oscillation) becomes. In the case of the conventional magnetron, the pulling figure is designed taking the operating stability into consideration. In this invention, the pulling figure is set in the range of 130 to 170% of that of the conventional magnetron to improve ease of use without degrading the operating stability.
(5) Resource-saving is another important item required for magnetrons used for home microwave ovens. Oxygen-free copper is used for anode cylinders and anode vanes of magnetrons. Molybdenum is used for filament leads. Both materials are expensive. It is therefore desirable to reduce the amount of those materials, and to reduce magnetrons in size and weight.
When the number of vanes is reduced, the anode loss per vane becomes large naturally, and the temperature of the vane rises excessively. As a result, adsorbed gas is discharged from the anode vanes, which deteriorates the degree of vacuum inside the tube, the strap rings connected with the tip of each vane are damaged, and the reliability of the magnetron is degraded. When' the number of vanes is reduced, therefore, the thermal allowance coping with the problems should be taken into consideration for design of the vane structure.
(6) The oscillation frequency band allowed for the microwave oven magnetron is 2450.apprxeq.50 MHz (2400 to 2500 MHz). The oscillation spectrum must be within this range.